Premium, high performance, lubricant basestocks generally exhibit useful viscosities over a wide range of temperatures, have improved viscosity index, and demonstrate lubricity, thermal and oxidative stability, and pour point equal to or better than conventional mineral oils. Such advantageous rheological and performance properties enhance their performance in lubricant formulations relative to mineral oil-based formulations, including a wider operating temperature window. However, premium lubricant base stocks are more expensive to produce than conventional mineral oil lubricants.
Many researchers have investigated ways of converting relatively low value hydrocarbon feedstocks, such as natural gas, into higher value products, such as fuels and lubricants. Additionally, much investigation has been conducted into catalytically upgrading waxy hydrocarbon feedstocks, which have significant concentrations of straight chain paraffin components, into more useful products by hydroisomerization and dewaxing, processes which isomerize and crack the straight chain paraffin wax components of the feedstock, respectively.
Processes for the production of hydrocarbon fuels and lubricants from synthesis gas, a mixture of hydrogen and carbon monoxide, have been known for some time, and of them, the Fischer-Tropsch (FT) process is probably the best known. An account of the development of the process and its more notable characteristics is given in Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, John Wiley & Sons, New York, 1980, Vol. 11, pp. 473-478.
In the FT process, synthesis gas, generally formed by partial oxidation of methane, is passed over a catalyst at elevated temperature and pressure to produce a number of carbon monoxide reduction products including hydrocarbons, alcohols, fatty acids and other oxygenated compounds. In favorable circumstances, oxygenated materials can comprise less than 1 percent of a total desired liquid product. The hydrocarbon product is highly paraffinic in nature and typically includes hydrocarbon gas, light olefins, gasoline, light and heavy fuel oils and waxy gas oils. Because the higher boiling fractions in the product are generally too waxy for general use either as liquid fuels or lubricants, further processing or upgrading is normally necessary before they can be used, either as such or by being added to the general pool of products. Advantageously, the FT products contain little, if any, of typical petroleum contaminants, such as aromatic compounds, cycloparaffinic compounds (naphthenes), sulfur compounds and nitrogen compounds, due to the relatively pure nature of the feedstocks: hydrogen and carbon monoxide, and ultimately, methane or natural gas.
U.S. Pat. No. 4,500,417 discloses conversion of the high boiling fraction of FT products by contact with a high-silica, large pore zeolite and a hydrogenation component to produce a distillate fraction and a lube fraction characterized by a high VI (viscosity index) and low pour point. Catalysts include zeolite Y, zeolite Beta, mordenite, ZSM-3, ZSM-4, ZSM-18 and ZSM-20.
U.S. Pat. No. 4,906,350 discloses a process for the preparation of a lubricating base oil with a high VI and a low pour point by catalytic dewaxing at least part of the hydrocrackate of a wax-containing mineral oil fraction over a zeolitic catalyst selected from among ZSM-5, ZSM-11, ZSM-23, ZSM-35, ZSM-12, ZSM-38, ZSM-48, offretite, ferrierite, zeolite beta, zeolite theta, zeolite alpha and mixtures thereof.
U.S. Pat. No. 4,943,672 discloses a process for hydroisomerizing FT wax to produce lubricating oil having a high VI and a low pour point by first hydrotreating the wax under relatively severe conditions and thereafter hydroisomerizing the hydrotreated wax in the presence of hydrogen on a particular fluorided Group VIII metal-on-alumina catalyst.
U.S. Pat. No. 5,059,299 discloses a method for isomerizing slack wax obtained from mineral oils and wax to form high VI, very low pour point lube oil basestocks by isomerizing over a Group VI-VIII on halogenated refractory metal oxide support catalyst, followed by solvent dewaxing.
U.S. Pat. Nos. 5,135,638 and 5,246,566 disclose wax isomerization processes for producing lube oil having excellent viscosity, VI and low pour point by isomerizing a waxy petroleum feed over a molecular seive having certain pore measurements and at least one Group VIII metal. Catalysts include SAPO-11, SAPO-31, SAPO-41, ZSM-22, ZSM-23 and ZSM-35.
U.S. Pat. No. 5,282,958 discloses a process for dewaxing a hydrocarbon feed including straight chain and slightly branched paraffins having 10 or more carbon atoms to produce a dewaxed lube oil using catalysts of a specified pore geometry and containing at least one Group VIII metal. The feedstock is contacted with the catalyst in the presence of hydrogen; exemplified catalysts include SSZ-32, ZSM-22 and ZSM-23.
U.S. Pat. No. 5,306,860 discloses a method of hydroisomerizing FT-derived paraffins over a series of catalysts, including a zeolite Y catalyst to form high VI, low pour point lube oils.
U.S. Pat. No. 5,362,378 discloses conversion of FT heavy end products with a platinum/boron-zeolite Beta catalyst having a low alpha activity to produce an extra high VI lube, which may then be dewaxed by conventional solvent dewaxing or by increasing the severity of the hydroisomerization step. European Pat. No. 0 776 959 A2 discloses a process for preparing lubricating base oils having a VI of at least 150 from a FT wax feed by first hydroisomerizing over a suitable catalyst in the presence of hydrogen and then either solvent or catalytically dewaxing the intermediate 390.degree. C.+ fraction.
However, none of the references discussed above disclose or suggest preparation of liquid hydrocarbons of a specific and limited range of compositions having any particular combination of branching properties, which lead to highly desirable lubricating properties including an unexpected combination of high viscosity index and low pour point. In fact, none of the cited references even discloses or suggests measuring Branching Index (BI) or Branching Proximity, as discussed below.
U.S. Pat. No. 4,827,064 discloses high VI synthetic lubricant compositions of polyalphaolefins wherein a "branching ratio", CH.sub.3 /CH.sub.2, is measured.
The disclosures of the U.S. patents described above are hereby incorporated by reference in their entireties.